Physical inactivity leads to detraining which includes loss of skeletal muscle mass (atrophy) and decreased aerobic capacity at the systemic and cellular levels. Hibernation is a natural example of extended inactivity and has received little attention. The applicant feels that this may be an important oversight in detraining studies because hibernators may have evolved the ability to resist manifestation of detraining. Significant impairment of locomotor capabilities after hibernation would reduce survival and reproductive fitness. Preliminary data on a species of hibernating ground squirrels, reported in this proposal for the first time, indicate that after normothermic detraining skeletal muscle mass is decreased but the mass-specific activity of aerobic enzymes is elevated. The fact they have "defended" their mitochondrial enzymes is profoundly different from results observed in detraining of nonhibernators. The proposed study is designed to increase the understanding of detraining in hibernators with three specific experiments: 1) to assess the degree of detraining in the golden-mantled ground squirrel (S. lateralis) hibernating at 5 degrees C; 2) to assess the effect of body temperature on this phenomenon by determining the degree of detraining when hibernating at a moderate temperature (20 degrees C); and 3) to determine whether the animals detrain differently when they are inactive at normal body temperatures by a comparison of detraining of these squirrels with a phylogenetically related and nonhibernating ground squirrel (the Antelope Ground Squirrel, Ammospermophilus leucurus) when both are detrained by restricting activity. The techniques used to assess detraining will include measurement of : 1) Whole animal performance: maximum exercise-induced rate of O2 consumption (VO2 max), maximum cold-induced rate of oxygen consumption (also know as summit metabolism or M max), and capacity for non-shivering thermogenesis. 2) Systemic changes: blood volume, whole blood hemoglobin, and resting heart rate. 3) Muscle cellular changes: activities of enzymes indicating aerobic, anaerobic and lipolytic capacity. The long-term objectives of this study are dictated by the interest in the effects of inactivity on physiological function. Inactivity may arise in a clinical setting (bed rest, casting, neurological damage, etc.) or non-clinical (space flight, for example). The examination of hibernators may offer significant insight into the mechanisms of detraining and, potentially, its prevention.